LPIC-2 Advanced Networking Configuration Practice Question
This LPIC-2 practice question tests your understanding of advanced networking configuration. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
You are a network administrator for a medium-sized company. The company has a Linux server acting as a router/firewall with three network interfaces: eth0 (public, 203.0.113.2/24, gateway 203.0.113.1), eth1 (DMZ, 10.0.1.1/24), and eth2 (internal, 192.168.1.1/24). The DMZ hosts a web server with IP 10.0.1.100, and the internal network has client machines. The firewall rules are currently set to default DROP on INPUT and FORWARD, ACCEPT on OUTPUT. The administrator wants to allow internal clients (192.168.1.0/24) to access the web server (10.0.1.100) on port 80. Additionally, the administrator wants to allow external users (from the internet) to access the web server's public IP (203.0.113.2) on port 80, which should be DNATed to 10.0.1.100. The administrator has enabled IP forwarding and added the following rules:
Internal clients cannot access the web server. External users are able to access the web server successfully. What is the most likely reason internal clients cannot access the web server?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The DNAT rule does not apply to traffic coming from the internal network because it is not received on eth0.
The DNAT rule is placed in the PREROUTING chain of the nat table, which only processes packets arriving on the specified interface (eth0). Internal clients (192.168.1.0/24) sending traffic to the web server (10.0.1.100) do not enter via eth0; they are routed through eth2. Therefore, the DNAT rule never matches their packets, and the destination remains 10.0.1.100, which is directly reachable without NAT. However, the FORWARD rules only allow traffic from eth0 to eth1 and back, so internal client traffic from eth2 to eth1 is implicitly dropped by the default FORWARD policy, causing the failure.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The DNAT rule does not apply to traffic coming from the internal network because it is not received on eth0.
Why this is correct
Internal traffic uses eth2 as input interface, so DNAT rule does not trigger; the destination remains the public IP, which is not local.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The web server cannot route back to the internal clients because it is in the DMZ.
Why it's wrong here
The web server can route back via the firewall because the firewall is the default gateway for both DMZ and internal.
✗
The FORWARD rule for return traffic does not match because the destination is the client.
Why it's wrong here
The rule for return traffic matches on source port 80, which is correct.
✗
IP forwarding is not enabled for the internal interface.
Why it's wrong here
IP forwarding is kernel-wide, not per interface.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume DNAT rules apply to all incoming traffic regardless of interface, but PREROUTING rules are interface-specific, and internal traffic bypasses them entirely.
Detailed technical explanation
How to think about this question
In Linux netfilter, the PREROUTING chain of the nat table is traversed before the routing decision, but only for packets entering the system. For internal clients, the packet enters via eth2, so it hits the FORWARD chain directly after routing, not PREROUTING. The administrator must add a FORWARD rule for traffic from eth2 to eth1 on port 80, and optionally a SNAT or MASQUERADE rule if the web server needs to send responses back through the router (though in this case, the web server's default gateway is likely the router's eth1 IP, so return traffic works without NAT).
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Advanced Networking Configuration — This question tests Advanced Networking Configuration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The DNAT rule does not apply to traffic coming from the internal network because it is not received on eth0. — The DNAT rule is placed in the PREROUTING chain of the nat table, which only processes packets arriving on the specified interface (eth0). Internal clients (192.168.1.0/24) sending traffic to the web server (10.0.1.100) do not enter via eth0; they are routed through eth2. Therefore, the DNAT rule never matches their packets, and the destination remains 10.0.1.100, which is directly reachable without NAT. However, the FORWARD rules only allow traffic from eth0 to eth1 and back, so internal client traffic from eth2 to eth1 is implicitly dropped by the default FORWARD policy, causing the failure.
What should I do if I get this LPIC-2 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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